Sulfur-containing polymers are known to be well-suited for use in aerospace sealants due to their fuel resistant nature upon cross-linking. Commercially valuable sulfur containing polymers have sulfur present either as polysulfide or as polythioether linkages in the polymeric backbone. Further, to impart flexibility and liquidity, these commercially available materials incorporate oxygen either as a formal linkage or as a simple ether linkage. Particular among these materials are polythioether polyether polymers which exhibit superior thermal resistance and flexibility as compared to the polysulfide polyformal materials.
U.S. Pat. No. 4,366,307 to Singh et al. teaches the preparation of liquid polymers having excellent low temperature flexibility (low glass transition temperature T.sub.g). The disclosed liquid polymers have pendent alkyl side chains and are produced by the acid-catalyzed condensation of hydroxyl-functional thioethers. The hydroxyl groups are in the .beta.-position with respect to the thio group for increased condensation reactivity. However, the disclosed condensation reaction has a maximum yield of about 75% of the desired condensation product. Furthermore, the acid-catalyzed reaction of .beta.-hydroxysulfide monomers yields significant quantities (typically not less than about 10%) of thermally stable and highly malodorous cyclic byproducts, such as 1-thia-4-oxa-cyclohexane. As a result, the commercial viability of the disclosed polymers is limited. The disclosed polymers also have a fairly high oxygen content, which results in water sensitivity. In addition, these polymers contain high levels of residual acid catalyst. These two factors yield materials having high electrical conductivity.
Morris et al., U.S. Pat. No. 4,609,762, describe reacting dithiols with secondary or tertiary alcohols to afford liquid polythioethers having no oxygen in the polymeric backbone. This approach reduces but does not eliminate the production of cyclic by-products. Further, by using hydroxyls of mixed reactivity, other competing side-reactions occur, such as internal dehydration to produce unsaturation and hence non-random polymerization. The disclosed polymers are less susceptible to water ingress due to their low oxygen content. However, their electrical conductivity is relatively high, again due to the presence of residual acid catalysts.
Certain high-resistivity (low-conductivity) sulfur-containing polymers and other non-sulfur containing polymers, such as polyurethanes, are also employed as "potting" compounds for applications such as coating electrical circuit boards to protect them from environmental damage. Known potting compounds suffer from a number of disadvantages. For example, lead peroxide-cured polysulfides have some fuel resistance but essentially no thermal resistance, and in addition are toxic due to the presence of residual lead oxide. Polysulfides cured using MnO.sub.2 rather than PbO.sub.2 avoid the toxicity problems associated with lead peroxide-cured polymers, but have reduced electrical resistivity. Polyether polyurethanes, on the other hand, have high electrical resistivity when cured but have essentially no fuel resistance. Use of two different layers to overcoat articles, with the first, underlying layer having high electrical resistivity and the second, overlying layer having high fuel resistance, has been proposed to address the foregoing problems.
A need exists for materials meeting the general performance requirement for aerospace sealants, such as room temperature liquidity, fuel resistance, and low temperature flexibility, and at the same time having minimal odor and high electrical resistance.